Accuracy vs. Precision

It is important to understand that measurements
(observations) may be precise but not accurate if they are closely
grouped together around a value that is different from the expectation
("true" value) by a certain amount. Also, observations may be
accurate but not precise if they are well distributed about
the expected value but are significantly dispersed from one
another. Finally, observations can be both precise and accurate
if they are closely grouped around the expected value (the distribution
mean).